Abebe Bikalia was running when they weren't even drinking water during the race. They didn't know squat about training compared to what they know now.
Also the 400 and 800 are the specific events that are the most dominated by doped athletes and intersex athletes, meaning that men have the biggest advantages in those races. Still, there are outliers in the sprints, where Shericka Jackson has repeatedly broken Paul Layton's 200m WR from 1962. Marathons have always been much closer to parity. Women have been running 1950s men's WR times since the 90s, so it's no surprise that after two decades, they've beaten 1960s men's times. Yes, shoes, training, and possibly doping have improved, but you're imagining a world in which everything is static except for EPO (which has been around for quite a while and was much more difficult to detect 30 years ago).